Fix #1531 (white background on transparent cover instead of black one)

cps/uploader.py

@@ -35,7 +35,7 @@ except ImportError:
     lxmlversion = None
 
 try:
-    from wand.image import Image
+    from wand.image import Image, Color
     from wand import version as ImageVersion
     from wand.exceptions import PolicyError
     use_generic_pdf_cover = False
@@ -149,6 +149,9 @@ def pdf_preview(tmp_file_path, tmp_dir):
             img.options["pdf:use-cropbox"] = "true"
             img.read(filename=tmp_file_path + '[0]', resolution=150)
             img.compression_quality = 88
+            if img.alpha_channel:
+                img.alpha_channel = 'remove'
+                img.background_color = Color('white')
             img.save(filename=os.path.join(tmp_dir, cover_file_name))
         return cover_file_name
     except PolicyError as ex: